Method of treating semiconductor devices



United States Patent METHODVOF SEMICONDUCTOR .DEVICESv ti`9n,lhiladelphia,l?a., a corporation of Pennsylvania NDrawing. Application February 25, 1,957 Serial N0. 641,805

4 invention relates to, the manufacture of. semicon-V dugtordevices, `andmore,particularly to the treatment of Sllhdevces preparatory'to the, soldering of the leads theieto In .some instances, the electrodes of semiconductor devices are not readily solderable, and therefore do not readily` lend themselves to thesoldering of leadsthereto. example of Such devices is the silicon alloy transistorv which aluminum is, alloyedg'with the surface of a siliconbody. In thelpast, a jet plating process was used to plate the aluminumfsiliconeutectic electrodes with a readilylsolderable, material,` such as nickel, without de= positing such materialen the silicon body.A However, thejet plating process, awhile.4 very well suited forsorne pin'po-ses,v isnot entirely satisfactory for the selective pltiileefthe .electrodes .of semiconductor devices- The fPlBCiPal object. ofv the present,V linvention is' fo' provide a simple, inexpensive and eilicient` process for theysele'c'tive plating of' suchelectrodes'. The process pxrovided by this inventionI is rnuch ysimpler and; less ex# pensive than theiet plating Processfor this purpose i'nventi'onis4 based .on the discoverythat if the surfaceof a semicondnctor. body' is coated with a rela- .tlvel'y.thckoxidevlmg the acts as ,a mask or barand: precludesdeposition on; said body of a readily solderablematerifali VIt wasfound that during the usual PIQCeSSingoof'a silicon alloytransistor a relatively thick oiriiefilm forms, on thesurface, and lthat thisv film pre'- cl'udesdhefelectroless deposition on the silicon body of areadi l,ylsolderabl e material' such as nickel. Followingthis .dig'scover` I conceived. the possibility that selec-v tive'plating of theelectrod'es Icouldl be achieved by simple immersion ofthe. ytransistor in a plating bath containing readily solderable material, while the oxide film is present on the semiconductor body; Experimentation provedfthis process tobe not only practical but far superior in every way ,to the. prior jet plating' process.

In thepractice of this invention it'is necessary (l) that-there be a suiciently thick oxide film on the semi'- conductorvbody, (2) that'. the electrode surfaces be conditionedto receivethe plating material, and (3) that thestemperature of the plating bath and the immersion time be such as. to effect the desired plating ofthe elecrode surfaeesbutnot to Vdissolve the oxide film on the semconduetcr body.

While'no` ,limitation ofthe invention `is intended, its use inconnectiqnwith silicon .alloy transistors has shown it to be very welll suited therefor, and it, will be described herein with particular reference to such use;

Firstwith. respect to therprovision of a sufficiently thick oxide film on the semiconductor body, as already mentioned, the usual processing of a silicon alloy transistonresnlts in the formation of an oxide film whichis ksufficientlythick for the purpose of this'- invention. HoweverLin any case where such a filmis not already provided,'it canybe readily produced byexposingthe silicon '2 example, the nlm may be produced smply'by exposingth e silicon surface toA an air ambient at 500 C; for'ore' nllLlte.

Next, with respect to conditioning the electrode faces yto receive the plating material, this is necessary in orderv to provide complete' rcoverage and good adhe= sion. The preferred procedure is to abrade the electrode surfaces, an'd then dip the device in a hydrochloric acid bath and rinse it in water, to remove any film fron' said surfaces. Of course, abrasion of the silicon surface' would tend to defeat the purpose of this invention, and therefore .such abrasion must be avoided. Th'e abrasion of the electrode surfaces' mayl be accomplished by scratching the surfacesiwith a fine metal probe ina larid"i operation, or it may4r be doney in suitable automatic' equipment; o l

An alternative procedure'for conditioning the elc# frode surfaces simply involves'preferential etching-ofthe surfaces with' hydrochloric acid and th'en rinsing.

Still another procedure involves preferentialJ etching ofthe electrode surfaces, followed by chemical depo'si fiony` of zinc thereon. It has been foundthat the suffiv sequent bath' plating proceeds at a' faster rate on ay Zinc treated surface.. However, prolonged"exposure` in' a zinc plating bathshould be avoided, as it may remove the* oxide film on the .semiconductor body and mav're'sulfin plating thereon. An example of a satisfactory iinc plating`-bath and `schedule isl .as follows:

While in some instances etching may be utilizedfo rernove film" from theelectrode surfaces; as in thelast v two procedures abovey mentioned, it should 'benote'd thai.l

. prepare said surfaces forY plating' will also tend :to remove surface: to; ambients such as oxyg enoxygen andwater, and nitrogen and water at elevated temperatures;` For die oxide nlm from the silicon body of the semicon-v ductor device. It iss-for this reason-that it ispreferred tox abrade the'v electrode surfaces tof loosen a'nyfil'r thereon, and their utilize an etch Whieliwill remove-"Itho filrn' from said surfaces Without appreciably affectingthe oxide' film on theisiliconbody.

Finally, with r'esp'e'ctto the-path plat-ing offreali-ly solderable materia-l, this' `should `be done within-a short time aftertlie conditioning of the electrode surfaces while the latter are free of any' fil'r'n'. Experimenta-tion with silicon alloy transistors has shownthat an alkaline plating' bath" ispreferable toanacid plating bath,-asf with an' acidl hairline-*time andte'mperatur'e conditions' aroquite critical and there is a tendencyv to produce a` flaky'del posit, with the aluminum being converted to aluminum hydroxide;

In the case of silicon alloy transistors, the conventional alkaline plating solution rgiven below has been found' to bevery satisfactory'.v

at a temperature of approximately 92 C., has been found to give excellent results, although a temperature range of 90 C. to 97 C. and an immersion time range of 1/2 minute to 21A. minutes have been found to produce satisfactory results.

The ammonium hydroxide is added until the pHis between 8.0 and 9.0. Within two minutes after the addition of the ammonium hydroxide, the semiconductor devices whose electrodes are to be plated are immersed in the solution preferably for two minutes. They are then removed and are rinsed in running water to remove the plating salts. Preferably they are finally rinsed in ethyl alcohol for approximately thirty seconds to remove any remaining salts and to hasten drying.

From the foregoing description, it will be seen that this invention provides a novel method of selectively plating an electrode or electrodes of a semi-conductor device with a readily solderable material for which the semiconductor body and the electrodes both have affinity, which method is particularly characterized in that it comprises immersing the device in a bath containing said material while said body is coated with an oxide lm of sufficient thickness to preclude deposition of said material on said body. 1t will be understood, of course, that any suitable plating material may be employed. Thus, while nickel has been mentioned in connection with the plating of aluminum-silicon eutectic electrodes of a silicon alloy transistor, materials such as cobalt and cobalt-nickel alloys can also be used for this purpose.

As previously mentioned, the method according to this invention is much simpler and less expensive than the previously employed jet plating process. By this method, the plating of electrodes of semiconductor devices can be performed in a batch operation, it being possible to bathplate a substantial number of devices at the same time. This method eliminates the need for complex and expensive equipment such as employed in the jet plating process.

Another important advantage of this method is that it produces a better product due to the excellent adhesion of the plating to the electrodes. Comparative tests of transistors produced by this method and by the jet plating process have shown that a much better soldered joint results from the employment of this method. Tension tests have shown that the soldered joint resulting from the employment of this method is able to withstand much greater tensional stress than is the soldered joint resulting from the employment of the jet plating process.

While this invention has been described with particular reference to the plating of aluminum-silicon eutectic electrodes of silicon alloy transistors, this is not to be taken as imposing any limitation on the invention. As hereinbefore indicated, the invention is intended to be applicable to any semiconductor device whose electrode surfaces it is desired or necessary to plate with a readily solderable material. Moreover, the method according to this invention may be modified as required to adapt it to any particular application.

I claim:

1. In the manufacture of a semiconductor device having on the semiconductor body at least one electrode composed of a metal which cannot readily be soldered, a method of selectively plating said electrode with a readilysolderable metallic material for which said body and said electrode both have afnity, which method comprises providing on said body an oxide film of sufficient thickness to preclude deposition of said material on said body, conditioning said electrode to receive said material, immersing said device in a bath containing said material at a temperature and for a time sufficient to effect plating of said material on said electrode without dissolving said oxide film, during which immersion said oxide film is effective to preclude deposition of said material on said body, and finally rinsing and drying said device.

2. In the manufacture of a semiconductor device body, conditioning said electrode by abrasion to receive said material, immersing said device in a bath containing said material at a temperature and for a time sufficient to effect plating of said material on said electrode with-' out dissolving said oxide lm, during which immersion said oxide film is effective to preclude deposition of said material on said body, and finally rinsing and drying said device.

3. In the manufacture of a semiconductor device having on the semiconductor body at least one electrode composed of a metal which cannot readily be soldered, a method of selectively plating said electrode with a readily-solderable metallic material for which said body and said electrode both have afiinity, which method comprises providing on said body an oxide film of suicient thickness to preclude deposition of said material on said body, preferentially etching the surface of said electrode to condition it to receive said material, immersing said device in a bath containing said material at a temperature and for a time sufficient to effect plating of said material on said electrode without dissolving said oxide film, during which immersion said oxide film is effective to preclude deposition of said material on said body, and finally rinsing and drying said device.

4. In the manufacture of a semiconductor device having on the semiconductor body at least one electrode composed of a metal which cannot readily be soldered, a method of selectively plating said electrode with a readily-solderable metallic material for which said body and said electrode both have affinity, which method comprises providing on said body an oxide film of sufficient thickness to preclude deposition of said material on said body, preferentially etching the surface of said electrode, immersing said device in a solution containing zinc at a temperature and for a time sufficient to effect plating of the zinc on said electrode without removing said oxide film, rinsing and drying said device, immersing said device in a bath containing said material at a temeperature and for a time sufficient to effect plating of said material on said electrode without dissolving said oxide film, during which latter immersion said oxide film is effective to preclude deposition of said material on said body, and finally rinsing and drying said device.

5. In the manufacture of a semiconductor device having on the semiconductor body at least one electrode composed of a metal which cannot readily be soldered, a. method of selectively plating said electrode with nickel for which said body and said electrode both have afiinity, which method comprises providing on said body an oxide film of sufficient thickness to preclude deposition of nickel on said body, conditioning said electrode by abrasion to receive nickel, immersing said device in a solution containing nickel at a temperature within the range of C. to 970 C. and for a time within the range of 1/2 minute to 21/2 minutes, thereby to plate the nickel on said electrode without dissolving said oxide film, during which immersion said oxide film is effective to preclude deposition of the nickel on said body, and finally rinsing and drying said device.

References Cited in the file of this patent UNITED STATES PATENTS 2,169,098 Howe Aug. 8, 1939 2,662,957 Eisler Dec. 15, 1953 2,695,852 Sparks Nov. 30, 1954 2,757,104 Howes July 3l, 1956 2,781,577 Smellie Feb. 19, 1957 UNITED STATES PATENT OFFICE. CERTIFICATE OF CORRECTION Patent Non 2y906,64'7 September .29;7 1959 John Roscben It is hereby certified that error a ppears in the printed specification of the above numbered patent requiring correction and that the said Letters Patent should read4 as corrected below.

Column .2y line 48, for path" read mbatb we; column 3, line 17, for "semieconduotor" reed semiconductor n; column 4, line 5, for "temeperature" read temperature n; line 6l, for "9700 0., read 970 0 er,

Signe/d and sealed this 29th day of March 1960;,

(SEAL) Attest:

KARL Ho @CLINE ROBERT C. WATSON Attesting Ocer Commissioner of Patents 

1. IN THE MANUFACTURE OF A SEMICONDUCTOR DEVICE HAVING ON THE SEMICONDUCTOR BODY AT LEAST ONE ELECTRODE COMPOSED OF A METAL WHICH CANNOT READILY BE SOLDERED, A METHOD OF SELECTIVELY PLATING SAID ELECTRODE WITH A READILYSOLDERABLE METALLIC MATERIAL FOR WHICH SAID BODY AND SAID ELECTRODE BOTH HAVE AFFINITY, WHICH METHOD COMPRISES PROVIDING ON SAID BODY AN OXIDE FILM OF SUFFICIENT THICKNESS TO PRECLUDE DEPOSITION OF SAID MATERIAL ON SAID BODY, CONDITIONING SAID ELECTRODE TO RECEIVE SAID MATERIAL, IMMERSING SAID DEVICE IN A BATH CONTAINING SAID MATERIAL AT A TEMPERATURE AND FOR A TIME SUFFICIENT TO EFFECT PLATING OF SAID MATERIAL ON SAID ELECTRODE WITHOUT DISSOLVING SAID OXIDE FILM, DURING WHICH IMMERSION SAID OXIDE FILM IS EFFECTIVE TO PRECLUDE DEPOSITION OF SIAD MATERIAL ON SAID BODY, ANF FINALLY RINSING AND DRYING SAID DEVICE. 